This invention relates to the generation of chlorine and sodium hydroxide from sodium chloride by electrolytic means using an electrolytic cell having an anode compartment, a cathode compartment, and a cation-exchange membrane separating the two compartments. More particularly, this invention relates to apparatus for generating chlorine electrolytically in a continuous process for such purposes as the treatment of water in swimming pools, treatment of water in remote locations for drinking, and for other such relatively small-scale purposes. More specifically, this invention relates to a set of methods of operating a small-scale, membrane-type electrochemical chlorine generator. These methods may be applied to several different designs of generator in addition to the design described below. These methods promote safe and highly efficient chlorine generation and facilitate the addition of chemicals for controlling the pH of the treated water and for stabilizing the chlorine in it.
There is frequently a need for the generation of chlorine, for use as a disinfectant in water, on a scale of the order of one to one hundred pounds per day. Examples are the sanitizing of swimming pools, the treatment of small water supplies or sewage systems, the prevention of algae growth in cooling towers, etc. In most of these applications the pH of the water being treated must be kept within narrow limits, for example 7.5.+-.0.1 pH units. Since chlorine is an acid gas, its use must be coupled with the addition of a chemically equivalent quantity of a basic compound if the pH of the treated water is to be kept constant. The most commonly used base for this purpose is sodium carbonate (soda ash). A buffering agent (usually sodium bicarbonate) is frequently added to the water being chlorinated to stabilize the pH so that little change in the pH of the water results from small fluctuations in the flows of the chlorine and neutralizing agent when both are being added nearly continuously.
Chlorine is highly toxic, so its use in the form of the liquified gas (commonly sold in steel cylinders holding 75 pounds or more) presents a significant hazard. For this reason chlorine is also sold in the form of a concentrated solution of sodium hypochlorite (NaOCl, sometimes called "liquid chlorine" or swimming pool bleach) or as the solid calcium hypochlorite (CaOCl.sub.2, "solid chlorine" or bleaching powder). These substances, which are quite costly relative to liquified chlorine gas, usually contain, respectively, excess sodium hydroxide or calcium oxide. Hence their use increases the pH of the water to which they are added and necessitates the frequent, or continuous, addition of hydrochloric acid (HCl or muriatic acid) to keep the pH constant. A second form of solid chlorine, chlorinated cyanuric (or isocyanuric) acid, has less tendency to cause pH drift but is so expensive that only in small pools can it be used as the sole method of chlorination. (The use of cyanuric acid as a chlorine stabilizer is discussed below.)
Another alternative to the use of liquified chlorine gas is to generate chlorine on site by the electrolysis of a concentrated aqueous solution of sodium chloride (NaCl or salt) in a two-compartment electrolytic cell. The two compartments are separated with a liquid-tight seal by an ion-exchange membrane (typically one of the DuPont Series 300 Nafion membranes). In the anode compartment chlorine is formed by discharge of chloride ion while sodium ion and water molecules are transported through the membrane. In the cathode compartment hydrogen and sodium hydroxide (NaOH) are generated. Patents for such devices have been granted during the past decade or so to, among others, Lynn (U.S. Pat. No. 4,308,123 [1981]), Tucker (U.S. Pat. Nos. 4,500,404 [1985], 4,693,806 [1987] and 4,781,810 [1988]) and Wreath & Keller (U.S. Pat. No. 4,613,415 [1986]).
However, there remains a need for improved electrolytic chlorine generators.